Valve systems for controlling the flow of liquids and/or gases, such as compressed air, natural gas, oil, propane, or the like, in a process system are generally known in the art. These systems can employ control valves to prevent or inhibit pressure surges within the fluids that can otherwise cause damage to components or disrupt system function.
In one application, a compressor increases the pressure of air flowing in the process system under normal operating conditions. If demand decreases, such as when a downstream throttling valve is closed, the need for a portion of the air decreases rapidly, and the flow through the compressor decreases. When the flow decreases enough, the compressor enters an unstable condition in which gas flows backwards through the compressor from the outlet side to the inlet side. At this point, the flow of gas oscillates rapidly between forward flow and reverse flow. This phenomenon is known as surge and is undesirable because it puts undue stress on the compressor components, such as blades and bearings.
Surge is generally addressed by the placement of a control valve around the compressor that diverts flow from the outlet of the compressor to the inlet when the compressor is near an operating point at which surge occurs. A control valve must act quickly because surge is a fast, unstable flow phenomenon. Traditionally, control valves have been on/off devices. However, with the advances in automation software and electronics as well as increased compressor sizes, significant improvements in operating efficiency and reliability can be attained if the control valve is instead a throttling device. To fully realize the benefits of a throttling valve instead of an on/off valve and to protect the compressor against surge, the position of the valve must be controlled both quickly and accurately.
The position of the control valve is normally controlled by a positioner. Large volume actuators can take 10 or 20 seconds or more to open or close the control valve with a standard positioner. Such a valve positioner cannot operate at, or deliver adequate volumes of fluid for moving the actuator at, the desired response times for optimal antisurge.
To address this problem, a control valve assembly can incorporate volume boosters in conjunction with the positioner on a throttling control valve to increase the stroking speed of the actuator. Valve and actuator stroke speed can be amplified or increased 15 or 20 times utilizing one or more volume boosters.
While the use of volume boosters reduces the problem of slow response time, it exacerbates any asymmetric performance of the valve in response to the positioner. Asymmetry is where, for example, the actuator is under-damped (or overshoots) in the closing direction and is over-damped (or sluggish) in the opening direction. Volume boosters not only amplify actuator response or stroke, but also amplify the inherent asymmetry. Such asymmetry is particularly noticeable on larger volume actuators equipped with volume boosters.
In general, the positioner and valve system are generally operated with a low voltage electrical system. The positioner converts an electrical signal, such as 4 mA-20 mA, to a pressure output to signal the actuator. Due to the low current flow under which the positioner operates and internal characteristics of the positioner, the signal delivered by the positioner to close the control valve is sent faster than the signal to open the valve. Thus, there is an inherent asymmetry in the performance of the positioner. The use of boosters only exacerbates this problem.
Generally, in compressor antisurge valve applications, the control valve assembly is equipped with long-stroke actuators and an equal number of volume boosters on the top side and the bottom side (top and bottom ports or chambers, respectively) of the actuator. In one example, due to the inherent asymmetry of the positioner and the multiple volume boosters, the actuator response will be under-damped in the opening direction and over-damped in the closing direction. In a fail-open actuator, i.e. where the actuator opens upon a surge condition, overshoot in the closing direction may not be particularly desirable in many circumstances. For example, in a compressor antisurge application, overshoot in the closing direction can accidentally send a valve into a surge condition by closing the throttling valve too far or too fast or both. It is known to those having ordinary skill in the art that generally, the deleterious effects of the under-damped response are reduced by detuning the positioner so that the response in the closing direction is critically damped. However, this detuning results in a substantially over-damped response in the opening direction that creates a very sluggish response and is objectionable.
In general, control valve assembly performance is improved if the actuator operates with dynamic symmetry. For example, an actuator operates with dynamic symmetry if the dynamic response in the opening direction of the valve is substantially the same as the dynamic response in the closing direction.
Accordingly, continual improvements in the construction and/or operation of control valve systems and their associated components are desirable.
While the disclosure is susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined by the appended claims.